Method of toughening glass in an oil bath

ABSTRACT

Glass of thickness in the range 0.5 mm. to 2.5 mm. is toughened by heating the glass to a temperature near to its softening point and then quenched with a selected oil whose viscosity is in the range 300 to 1,000 centistokes at 38* C. and whose flash point is in the range 220* to 310* C., whilst maintaining the temperature of the oil in the range 150* to 240* C. There is thereby induced in the glass a ratio of surface compressive stress to central tensile stress in the range 1.5:1 to 3.5:1.

Umted States Patent 1151 3,640,694

Giddings et al. Feb. 8, 1972 [54] METHOD OF TOUGHENING GLASS IN [56]References Cited AN UNITED STATES PATENTS [72] Inventors: David GeorgeGiddings, Stourbridge; 1359215 5/ 1934 w Douglas 'l\vist, Hollywood,near Bib 2,198,739 4/1940 Phillips ..65/1 16 mmgham bmh England FOREIGNPATENTS OR APPLICATIONS [73] Assgnee Pdkmgm 316,]08 11/1956 Switzerland"65/11 England [22] Filed; D 30, 1969 Primary ExaminerArthur D. KelloggAttorney-lmirie and Smiley and Snyder and Butrum [211 App]. N0.: 889,341

[57] ABSTRACT Fore g Applicafiml Priority Data Glass of thickness in therange 0.5 mm. to 2.5 mm. is Jan. 8, 1969 Great Britain ..1,263/69 l byheating the glass 3 tefnpeamre June 24 1969 Great Britain ..3 195/69 9 9and quenched Selected viscosity is in the range 300 to 1,000 cennstokesat 38 C. and a a q t 52 US. Cl ..65/ll6 whse flash m the 220 310 C 5taining the temperature of the oil in the range 150 to 240 C. There isthereby induced in the glass a ratio of surface compressive stress tocentral tensile stress in the range 1.5:] to 3.521.

10 Claims, No Drawings METHOD OF TOUGHENING GLASS IN AN OIL BATHBACKGROUND OF THE INVENTION This invention relates to the toughening ofglass, and more especially to the manufacture of thin toughened glass.

This toughened glass may be used as a single sheet or as a laminatedglass assembly for the windscreen, back light or side lights of a motorvehicle.

SUMMARY In accordance with the invention a method of toughening glasswhose thickness is in the range 0.5 mm. to 2.5 mm., comprises heatingthe glass to a temperature near to its softening point, and quenchingthe hot glass with a selected oil maintained at a temperature in therange 150 to 240 C. to induce in the glass a ratio of surfacecompressive stress to central tensile stress in the range 1.5:1 to 3.5:l.

The selected oil has a viscosity in the range 300 to 1,000 centistokesat 38 C. and a flash point in the range 220 to 3 C.

The invention is particularly applicable to the toughening ofsoda-lime-silica glass that is heated to a temperature in the range 600to 740 C.

The invention also provides toughened glass whose thickness is withinthe range 0.5 mm. to 2.5 mm., and has a ratio of surface compressivestress to central tensile stress in the range 1.5:1 to 35:1.

The invention further provides a laminated glass assembly incorporatingat least one sheet of such toughened glass.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In each of the examples set outbelow a thin sheet of glass, for example 30 cm. X 30 cm. and of ordinarysoda-lime-silica glass composition, was toughened by quenching in aselected oil.

A preferred range for the initial glass temperature is 600 to 740 C. andit was found that the modulus of rupture of the glass depends on itsinitial temperature. Also, it has been found advantageous in preservingthe surface quality of the glass to work towards the hotter end of thistemperature range. However, a medium temperature in the range issuitable, and in a number of the examples the glass was heated to about700 C. and then lowered through as short a path as possible into a bathof chilling oil. In each example, the oil is a fresh clean oil and isheated and continuously circulated through the tank so that thetemperature of the oil is maintained in the range l50 to 240 C. andpreferably at 200 C.

The viscosity of the oil and its constitution have been found to have abearing on the stresses produced in the glass. The more viscous the oil,the higher the stress produced in the m or E A L glass. Generally themore viscous oils have a higher flash point and for the present purposesthe selection of the oil is defined in terms of its viscosity and itsflash point. Both these factors influence the initial boiling point ofthe first distillate from the oil and thus are an indication of theconstitution of the oils selected.

It has been found that the required modulus of rupture and centraltensile stress, as well as the required ratio of surface compressivestress to central tensile stress, can be produced by selecting oilswhose viscosity is in the range 300 to 1.000 centistokes at 38 C. andwhose flash point is in the range 220 to 310 C.

Prior to the heating of the glass it has been found advantageous toeffect a preliminary edge preparation of the glass and a bright edgefinish is given to the glass by first working the edge with a linishercarborundum belt and then finishing the edges of the glass with alinisher" belt of cork impregnated with ceri-rouge.

The sheet of thin glass to be toughened is suspended by means of tongsin a vertical heating furnace with an exit mouth in its lower face. Whenthe glass has been heated to an initial temperature within the range 600to 740 C., the tank of chilling oil which is maintained at a temperaturewithin the range to 240 C., is raised to be as near as possible to thelower mouth of the furnace and the suspended hot glass is lowered at acontrolled rate of 30 cm. per second into the oil. There is an initialsharp chilling of the glass with some fuming from the oil and thereaflerthe glass gradually cools to the temperature of the oil and the desiredstresses are produced in the glass by the time it reaches thetemperature of oil and can be removed for washing.

The table quoted below gives a number of examples of the stressesobtained by quenching a glass pane in a selected oil. In each example,the pane was 30X30 cm., which when it was heated to the initialtemperature given in the table, was lowered at a controlled rate of 30cm. per second into a tank of fresh oil maintained at the temperaturegiven in the table.

The thickness of the glass, the initial temperature of the glass and theoil temperature were varied to both extremes of the particular rangesquoted above. Also the selected oils comprise three different oils andthe particular oil used in each example is given in the table. The firstoil was Cylrex 200 M (Mobil Oil Company), a heavy naphthenic oil whoseflash point is 305 C. and whose viscosity is 974 centistokes at 38 C.The next oil was Cylrex FM (mobil Oil Company) which is a lighter oilhaving a flash point of 275 C. and a viscosity of 640 centistokes at 38C. The third oil, Vacuoline AA (Mobil Oil Company), is a lighter oilstill whose flash point is 225 C. and viscosity is in the range 310 to342 centistokes at 38 c.

The methods employed for determining the values of the modulus ofrupture, central tensile stress and ratio of surface Central Ratio ofsurface Thickness Type of Oil Modulus of tensile compressive of glassTemperature (Mobil Oil Temperature rupture in stress in stress tocentral Example No. in mm. of glass C. Company) of oil C. Kg/cm Kg/cmtensile stress l 0.5 740 Cylrex 200 M 150 1680 390 3:1 2 .5 700 CylrexFM 240 1450 300 2.5:1 3 .5 700 Vacuoline AA 200 1550 300 2.521 4 1 700Cylrex 200 M 200 1550 350 3:1 5 l 685 Cylrex FM 220 1680 390 3:1 6 1 720Cylrex FM 1750 4-20 3:1 7 l 660 Vacuoline AA 150 1550 320 3:1 8 1.8 650Vacuoline AA 150 1600 350 3.5:1 9 2 670 Cylrex 200 M 200 2000 4-50 3.51110 2 600 Cylrex FM 225 1300 285 1.5:1 ll 2 630 Cylrex FM 225 1500 3502:1 12 2 650 Cylrex FM 225 1700 400 2.511 13 2 675 Cylrex FM 220 1750420 3:1 14 2 700 Cylrex FM 200 1750 420 3:1 15 2 685 Vacuoline AA 1502000 450 3:1 16 2.25 650 Cylrex FM 225 2000 450 25:1 17 2.5 700Vacuoline AA 200 2100 460 3.511 18 2.5 650 Cylrex 200 M 240 2100 4603.5:] 19 2.5 650 Cylrex FM 200 2100 460 3.521

compressive stress to central tensile stress quoted in the followingtable of examples will now be described.

The toughened glass sheet under test was placed across a pair of knifeedges and a steadily increasing load was applied through a similar pairof knife edges placed near the midpoint of the glass sheet. The modulusof rupture, which effectively corresponds to the breaking stress in theconvex surface of the glass sheet when in tension, was then calculatedfrom the load applied at the instant of fracture and the cross sectionof the glass sheet in shear.

The central tensile stress in the toughened glass sheet was measured byan instrument developed by Triplex Safety Glass Company Limited which isemployed to pass a beam of polarized light at a grazing angle into aprincipal surface of the glass sheet in such a manner that the beamemerges through the median of the peripheral edge surface of the sheet.The beam leaving the peripheral edge is analyzed by a Babinetcompensator and the central tensile stress in the glass sheet is thenobtained by observing the slope of the fringes formed in the beam andcomparing this slope with a previous calibration.

Finally, the surface compressive stress of the glass sheet, forcalculation of the ratio of the surface compressive stress to centraltensile stress, is obtained by using the differential surfacerefractometer designed by the Pittsburgh Plate Glass Company. With thisinstrument a beam of polarized light is refracted into a principalsurface of the glass sheet at a critical angle which is predetermined bythe relative refractive indices of the instrument components and theglass sheet, the beam travelling close to and parallel with the surfaceof the sheet. The compressive stress in the surface of the sheet isrelated to the difierence in the refractive indices for light which ispolarized perpendicular and parallel to the plane of incidence, and thelight which emerges from the surface is analyzed to give a measure ofthe surface compressive stress in the glass sheet.

These examples show that the toughening of glass at the thinner end ofthe thickness range is more readily effected with a heavier or moreviscous oil. Similarly, the thicker the glass or the higher the initialtemperature, the greater is the modulus of rupture in the resultanttoughened glass.

Thus the invention provides an improved method of toughening thin glasshaving a thickness in the range 0.5 mm. to 2.5 mm.

The selection of particular oils for effecting the quenching of the hotglas, which oils are used in the fresh state and continuously circulatedthrough the quenching tank, has been found most effective in producingcontrolled toughening of the thin glass so there is accurately achievedin the glass the preferred central tensile stress in the range 285kg./cm. to 460 kg./cm. and the ratio of surface compressive stress tocentral tensile stress in the range 1.5:1 to 3.5:1. This ensures thenecessary strength of the glass to permit it to be incorporated in motorvehicles as component parts or to be used in locations, e.g., domesticglazing, where similar strength is required.

In particular the thin toughened glass produced by the method isadvantageous in a laminated windscreen. The outer sheet of thistoughened glass breaks without loss of vision if struck by a sharp stoneand when the inner sheet is impacted by a head the glass shatters intosmall particles which neither cause laceration of the occupants in thevehicle nor the interlayer of plastics material in the laminate.

We claim:

1. A method of toughening glass whose thickness is in the range 0.5 mm.to 2.5 mm., comprising heating the glass to a temperature near to itssoftening point, and quenching the hot glass with a selected oil whoseviscosity is in the range 300 to 1,000 centistokes at 38 C. and whoseflash point is in the range 220 to 310 C., while maintaining thetemperature of the oil below its flash point and in the range to 240 C.,thereby inducing in the glass a ratio of surface compressive stress tocentral tensile stress in the range 1.5: 1 to 3.5: l.

2. A method according to claim 1, wherein the oil induces in the glass acentral tensile stress in the range 285 kg/cm. to 460 kg./cm.

3. A method according to claim 1, wherein the glass to be toughened issoda-lime-silica glass and wherein the glass is heated to a temperaturein the range 600 to 740 C.

4. A method of toughening glass whose thickness is in the range 0.5 mm.to 2.5 mm., comprising heating the glass to a temperature near to itssoftening point, and quenching the hot glass with a selected oil whoseviscosity is in the range 300 to 1,000 centistokes at 38 C. and whoseflash point is in the range 220 to 310 C., while maintaining thetemperature of the oil below its flash point and in the range 150 to 240C., thereby inducing in the glass a ratio of surface compressive stressto central tensile stress in the range 2.5:! to 3.5: l.

5. A method according to claim 4, wherein the oil induces in the glass acentral tensile stress in the range 300 kg./cm. to 460 kg./cm.

6. A method according to claim 4, wherein the glass to be toughened issoda-lime-silica glass and wherein the glass is heated to a temperaturein the range 650 to 740 C.

7. A method of toughening glass whose thickness is in the range 0.5 mm.to 2.5 mm., comprising heating the glass to a temperature near itssoftening point, and quenching the hot glass with a selected oil whoseviscosity is in the range 300 to 1,000 centistokes at 38 C. and whoseflash point is in the range 220 to 310 C., whilst maintaining thetemperature of the oil in the range 150 to 200 C., thereby inducing inthe glass a ratio of surface compressive stress to central tensilestress in the range 2.5:1 to 3.521.

8. A method according to claim 7, wherein the oil induces in the glass acentral tensile stress in the range 350 kg./cm. to 455 kg./cm.

9. A method according to claim 7, wherein the glass to be toughened issoda-lime-silica glass and wherein the glass is heated to a temperaturein the range 685 to 740 C.

10. A method according to claim 1, wherein the glass is 2 mm. thick,comprising heating the glass to 700 C., quenching the hot glass in anoil whose viscosity is 640 centistokes at 38 C. and whose flash point is275 C., which oil is maintained at 200 C.

UNITED STATES PATENT OFFICE CERTEFICATE 0F CORRECTIGN Patent No.3,640,694 Dated Feb. 28, 1972 inventofls) David George Giddings andDouglas Twist It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Priority of British application "3,195/69" should be -3l95l/69 Signedand sealed this 29th day of August 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents USCOMM-DC 60376-P69 w u.s GOVERNMENT PRINTING OFFICE: 19590-365-334 FORM PO-IOSO (10-69)

2. A method according to claim 1, wherein the oil induces in the glass acentral tensile stress in the range 285 kg./cm.2 to 460 kg./cm.2.
 3. Amethod according to claim 1, wherein the glass to be toughened issoda-lime-silica glass and wherein the glass is heated to a temperaturein the range 600* c. to 740* C.
 4. A method of toughening glass whosethickness is in the range 0.5 mm. to 2.5 mm., comprising heating theglass to a temperature near to its softening point, and quenching thehot glass with a selected oil whose viscosity is in the range 300 to1,000 centistokes at 38* C. and whose flash point is in the range 220*c. to 310* C., while maintaining the temperature of the oil below itsflash point and in the range 150* c. to 240* C., thereby inducing in theglass a ratio of surface compressive stress to central tensile stress inthe range 2.5:1 to 3.5:1.
 5. A method according to claim 4, wherein theoil induces in the glass a central tensile stress in the range 300kg./cm.2 to 460 kg./cm.2.
 6. A method according to claim 4, wherein theglass to be toughened is soda-lime-silica glass and wherein the glass isheated to a temperature in the range 650* c. to 740* C.
 7. A method oftoughening glass whose thickness is in the range 0.5 mm. to 2.5 mm.,comprising heating the glass to a temperature near its softening point,and quenching the hot glass with a selected oil whose viscosity is inthe range 300 to 1,000 centistokes at 38* C. and whose flash point is inthe range 220* c. to 310* C., whilst maintaining the temperature of theoil in the range 150* c. to 200* C., thereby inducing in the glass aratio of surface compressive stress to central tensile stress in therange 2.5:1 to 3.5:1.
 8. A method according to claim 7, wherein the oilinduces in the glass a central tensile stress in the range 350 kg./cm.2to 455 kg./cm.2.
 9. A method according to claim 7, wherein the glass tobe toughened is soda-lime-silica glass and wherein the glass is heatedto a temperature in the range 685* to 740* C.
 10. A method according toclaim 1, wherein the glass is 2 mm. thick, comprising heating the glassto 700* C., quenching the hot glass in an oil whose viscosity is 640centistokes at 38* C. and whose flash point is 275* C., which oil ismaintained at 200* C.